In the 3rd generation partnership project radio access network long term evolution (3GPP LTE), a technique for increasing the capacity of a system using a small cell in addition to a macro cell has been discussed. A “cell” is an example of a wireless area that is formed according to the coverage of radio waves transmitted by a wireless base station. A wireless apparatus, such as a mobile station, in a cell can wirelessly communicate with a wireless base station forming the cell.
The name of the cell may vary depending on the size of the coverage. It may be considered that the size of the coverage varies depending on the maximum transmission power of the wireless base station. For example, the “macro cell” has a coverage wider than that of a “small cell”.
In 3GPP LTE-Advanced (LTE-A), a wireless communication system having a configuration in which one or a plurality of small cells are arranged so as to overlap a macro cell (which may be referred to as an “overlay configuration”) has been examined. The wireless communication system (which may be referred to as a “wireless network”) having the above-mentioned configuration may be referred to as a “heterogeneous network”.
In the heterogeneous network, a technique in which a mobile station is connected to both a macro cell and a small cell and communicates with the cells has been examined. In addition, a technique in which a mobile station is connected to two different small cells and communicates with the cells has been examined. As such, in some cases, the aspect in which the mobile station is connected to two different small cells and communicates with the cells is referred to as dual connectivity (DC).
In the dual connectivity in which a mobile station is connected to both a macro cell and a small cell, a control plane signal including the control information of layer 3 for, for example, setting a transmission path (which may be referred to as a “path”) or controlling handover (HO) is transmitted and received between the base stations forming the macro cell. In contrast, for example, a data plane signal including user data is transmitted and received between the base station forming the macro cell and the base station forming the small cell.
The control plane may be referred to as, for example, a C plane or a signaling radio bearer (SRB). The data plane may be referred to as, for example, a user plane (U plane) or a data radio bearer (DRB).
The base station to which the control plane is connected may be referred to as a “primary base station”. The base station which performs communication in cooperation with the primary base station and to which the data plane is connected may be referred to as a “secondary base station”.
The primary base station is referred to as an “anchor base station” or a “master base station” and the secondary base station is referred to as an “assisting base station” or a “slave base station”. In the recent trend of LTE-A, the terms “master base station” and “secondary base station” are used.